Karstedt’s complex

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Complexes of the type 48-53 (Scheme 2.7) have been targeted as pre-catalysts for the hydrosilylation of alkenes [44]. For example, in the hydrosilylation of 1-octene with (Me3SiO)2Si(Me)H, which was studied in detail as a model reaction, the activity of complexes 48-49 with alkyl substituted NHC ligands, is inferior to that of the Karstedt s system. However, selectivity and conversions are dramatically improved due to the suppression of side-product formation. In this reaction... 

The hydrosilylation of alkynes has also been studied using as catalysts Pt, Rh, Ir and Ni complexes. The improvement of the regioselectivity of the catalyst and the understanding of stereoelectronic factors that control it have been major incentives for the ongoing research. From numerous studies involving non-NHC catalysts, it has been established that there is a complex dependence of the product ratio on the type of metal, the aUcyne, the metal coordination sphere, the charge (cationic versus neutral) of the catalytic complex and the reaction conditions. In the Speier s and Karstedt s systems, mixtures of the thermodynamically more stable a- and -E-isomers are observed. Bulky phosphine ligands have been used on many occasions in order to obtain selectively P-f -isomers. 

Combining two different hydrosilylation catalysts in sequence with chlorodimethylsilane has allowed the construction of complex dye assemblies and conducting polymers. In the example shown (Scheme 8), Karstedt s catalyst was chosen after a brief screen for the hydrosilylation of an aromatic diyne with chlorodimethylsilane. After reduction of the chlorosilane, an equimolar mixture of disilane B and diyne A was treated with a catalytic amount of Wilkinson s catalyst, resulting in the formation of polymer C.42 Hydrosilylation of alkynes has also been studied as a means of synthesizing oligo(phenylenevinylene) units with pendant alkoxysilanes to create curable, hole-transporting films.43,43 ... 

Karstedt s catalyst, nickel analog, 8, 138-139 Kedarcidin, via ring-closing diene metathesis, 11, 208-209 Kendomycin, via ring-closing metathesis, 11, 239 KERD, see Kinetic energy release distribution Ketenes, niobium complexes, 5, 84 Ketenimines... 

Vinyltrimethylsilane (97%), trimethylchlorosilane (98%), chlorodimethylsilane (97%), bromoform (96%), 5-bromo-l-pentene (95%), nBuLi (2.5M solution in hexanes), MeLi (1.6M solution in diethyl ether), n-decane (puriss. p.a., standard for GC, > 99.8%) and hexachloroplatinic(lV) acid hydrate were purchased from Aldrich. Platinum divinyltetramethyldisiloxane complex (Karstedt s catalyst, 3% solution in xylenes), hexamethyl-cyclotrisiloxane (95%), vinylmethyldichlorosilane (97%) and 1,1,3,3-tetramethyldisiloxane (97%) were bought from ABCR. Bromine (puriss) was bought from Eluka. Triethylamine (pure for analysis) and zinc oxide (pure) was purchased from Chempur. Solvents (tetrahydrofurane, diethyl ether, methylene chloride, pentane, ethyl acetate) were supplied by POCh (Polish Chemical Reagents). 

Pt(0) (vinylMe2Si-0-SiMe2vinyl)i 5 Karstedt s catalyst, platinum divinyltetramthyldisiloxane complex Scheme 1 Homogeneous platinum catalysts... 

At last, Hamaide et al. described also shortly their attempts for performing directly the hydrosilylation without protective groups. In addition to incompatible solubilities, they observed up to 50% 0-silylation, instead of C-silylation with Karstedt s catalyst (Platinium divinyltetramethyldisiloxane complex) [21]. 

T-Olefin platinum(O) complexes are important starting materials for oxidative addition see Oxidative Addition) or catalysts. Karstedt s catalysts, which are the most active ones for hydrosilylation, have been structurally characterized and found to show the structure of Pt2(M y M y )3 (9), wherein = divinyltetramethyldisiloxane." A styrene analogue Pt°(styrene)3 provides a convenient route to get an r-alkyne platinum complex by displacement (Scheme 27). DFT calculations indicate that aUcyne in the... 

NHC-Pt(O) complexes were synthesized from Karstedt s catalyst (99) (equation 13). Substitution of DVTMS by two equivalents of in situ generated NHC led to the formation of a series of [(NHC)Pt(DVTMS)] complexes (100). The first time, this was developed for ICy, TBu, and IDM subsequently, this synthetic procedure was efficiently applied to numerous imida-zolylidene and benzimidazolylidene derivatives independent of their V-substituents. ... 

Most research and industrial syntheses are carried out in the presence of platinum complexes, with H2PtCl6 commonly used as the initial precursor. A solution of this catalyst in isopropanol (1-10%) is referred to as Speier s catalyst [11]. In addition to isopropanol other solvents (alcohols, ketones, aldehydes, ethers, esters, THF, hydrocarbons) have also been used in the preparation of active catalysts from chloroplatinic acid. Since 1957 hundreds of catalysts based on chloroplatinic acid and other d -Pt and d °-Pt° complexes have been reported [1-5]. The Karstedt s type of catalyst obtained by treating hexachloroplatinic acid with divi-nyldisiloxane was discovered in 1973 [33] and has predominated in recent years. This catalyst has the empirical formula Pt2(CH2=CHMe2SiOSiMe2CH=CH2)3. Its structure was reported [34, 35]. 

Transition-metal-catalyzed hydrosilylation was first reported in the late 1950s with catalysts based on platinum, ruthenium, and iridium chlorides. For industrial applications, chloroplatinic acid (H PtCl nHjO) has been used extensively and is highly active for this process. This catalyst has become known as Speier s catalyst. This catalyst is spectacularly reactive, as indicated by the low catalyst loading for the reaction in Equation 16.17. A Pt(0) complex containing vinylsiloxane ligands (platinum divinyltetramethyl-disiloxane) shown in Figure 16.1 has also been used frequently in industrial settings as a catalyst for hydrosilylation. Tliis catalyst has become known as Karstedt s catalyst. ... 

In the Chalk-Harrod mechanism, oxidative addition of silane occurs to a Pt(0) complex. The ancillary ligands on the active catalyst when reactions are initiated with "Speier s catalyst" are unknown, but are likely to be the olefin substrate. The ancillary ligands on the active form of Karstedt s catalyst or the related PtfCOD) complex could be the original diene ligands or the olefin substrate. Details on the mechanism of oxidative additions of silanes are provided in Chapter 6. In brief, this reaction occurs by coordination of silane to an open site to form a silane a-complex, followed by cleavage of the Si-H bond to form a silyl hydride species. 

Transition metal complex catalyzed ring-opening polymerization is fairly general and many other types of ferrocenylsilanes have also been polymerized by this methodology. For example, ferrocenophanes containing acetylide substituents as well as etheroxy substituents have been polymerized by the use of Karstedt s catalyst (platinum-divinyltetramethyldisiloxane complex) (Fig. 8.25) [56]. 

Many Pt(0) complexes being derivatives of Karstedt s catalyst have also been synthesized, and quite a few of them, such as complexes with various phosphines and quinines, have proved selective and efficient for hydrosilylation of alkenes. 

A comprehensive study on homoleptic dinuclear Pd(0) compounds gives significant results especially for the operation of naked Pd(0) and L-Pd(O) complexes in homogeneous catalysis (63). It seems quite possible that these complexes (e.g., Pd analogue of Karstedt s catalyst, with tetramethyldivinyldisiloxane ligand) would have replaced Karstedt s catalyst in the hydrosilylation processes. 

Early concepts regarding the extremely efficient hydrosilylation catalysis by chloroplatinic acid involved the reduction of the complex to colloidal platinum, which was believed to be the real catalyst. However, the reports by Lewis and co-workers (e.g., (131,132)) have shown that the processes leading to colloid formation can account for the high activity of a hydrosilylation catalyst based on the complexes of Pt(0) with olefins and dienes such as divinyltetramethyldisiloxane (Karstedt s catalyst) and cyclooctadiene. The reactivity order of platinum group... 

Hexachloroplatinic acid and other platinum complexes are mainly used as soluble catalysts for the additive cure. The most active catalyst used recently for vulcanization of silicon rubber is the platinum-alkenylsiloxanes complex, mainly the platinum-vinylsiloxane complex (Karstedt s catalyst) (4). One important approach to the activated cure of silicone rubber makes use of various inhibitors or moderators added to the platinum catalyst to reduce, or temporarily inhibit, its catalytic activity in the presence of the alkenyl- and hydropolysiloxanes (see catalysis by Pt complexes). The catalyst is usually added to the reaction mixture in quantities related to the number of unsaturated (e.g., vinyl) substituents in the polysiloxane. Vinyl-terminated polydimethylsiloxane polymers (viscosity > 200 cSt) are typically cross-linked by methylhydrosiloxane-dimethylsiloxane copolymer with 15-50 mol% of polymethylhydrosiloxane. A typical catalyst is a platinum complex in alcohol, xylene, divinylsiloxanes, or cyclic vinylsiloxanes. The system is usually prepared in two parts (part A, vinylsiloxane -I- Pt (5-10 ppm) part B, hydrosiloxane -I- vinylsiloxane). Inhibitors stop the platinum catalyst they are volatile or react with silicone hydride cross-linker to become a part of the polymer network. Some of them are decomposed by heat or light (UV). A single-component system contains fugitive inhibitors of Pt. 

In most cases of hydrosilylation in polymer systems, the role of catalysts is played by transition metal complexes, those of platinum in particular, mainly Speier s and Karstedt s catalysts (3,4,6,12,18). Rhodium complexes also play an important role. They show higher resistance to poisoning than by platinum complexes (7). In this group of catalysts, one of the most efficient catalysts in... 

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